Patent Document 1 discloses a technique related to a bearing structure in which a rotational shaft of an internal combustion engine is supported by a radial plain bearing via lubricating oil. In Patent Document 1, a PTC heater is provided opposing an outer peripheral surface of a radial plain bearing mounted on a cylinder block and a bearing cap of an engine, and a thermal insulation material is provided on a side, of the PTC heater, opposite (outer peripheral side) to the side opposing the outer peripheral surface of the radial plain bearing. During a cold start of the engine, the radial plain bearing is heated by the PTC heater such that the lubricating oil filled between the radial plain bearing and a crank shaft is heated. With this configuration, viscosity of the lubricating oil filled between the radial plain bearing and the crank shaft is reduced, and a viscous frictional loss when the crank shaft is rotated is reduced.